Commercially available image display systems in the medical field utilize various techniques to present medical images to a medical practitioner. Specifically, the images produced within modalities such as Computed Radiograph (CR), Magnetic Resonance Imagery (MRI) and the like are displayed on a display terminal for review by a medical practitioner at a medical treatment site. These medical images are displayed on a video monitor and the displayed images are used by the medical practitioner to determine the presence or absence of a disease, tissue damage etc.
Medical images, whether directly acquired from digital imaging modalities or from scanning films, are typically recorded with various intensities. Each dot (pixel) may be represented by a particular pixel intensity value. The acquired medical image may not always be satisfactory for radiologists' studies. For example, some medical images may be underexposed or overexposed. In order to improve readability the image brightness and/or contrast may need to be adjusted. Out of a number of various pixel intensity values, there is usually a certain range of pixel intensities values that is most useful for viewing any particular medical image or study.
The technique of window processing has been developed to improve the diagnosis of a region of interest in a diagnostic image. Because the tonal range of a region of interest may be small compared to the tonal range of the entire digital medical image, insufficient contrast in the region of interest may inhibit proper diagnosis. By expanding the tonal range in the region of interest to the entire tonal range of the display device through windowing, image contrast in the region of interest is greatly enhanced. Proper diagnosis is therefore greatly facilitated.
A particular pixel intensity value range of interest is defined as a “window”. The distance between the two pixel intensity values at the two ends of the intensity range is called the “window width”. As shown in FIG. 1, the “window width” is the range of pixel intensity values in the input digital medical image that are selected to be displayed over the full tonal range of the output display device. The intensity of “uninterested” pixels with values outside of the window are either mapped to black (i.e. fully underexposed) or white (i.e. fully overexposed). For example, in FIG. 1A, the window range is between 80 and 600. The pixel value of 100 is within the window, while the value of 40 is outside of the window and therefore would be mapped to “black”. The pixel value of 700 is also outside of the window and would be mapped to “white”.
The intensity of each medical image has a certain distribution. For example, tissues with higher density may produce a high number of “dark” pixels while less dense tissues will be more “bright” in the image. Unfortunately, the human eye is limited in its ability to discern intensities that are too similar. A “window level transformation” is a mathematical transformation that defines how to map pixel values within the window into display luminance values.
For example, if a medical practitioner wants to study a dense tissue area and wants to see as much detail as possible, it may be desired to select a level function like the one shown in FIG. 1. Even though pixel values 140 and 141 have a pixel intensity value difference of 1, after level function mapping, they will have a luminance difference of 5 when displayed. Human eyes will not be able to differentiate between two dots with pixel intensity values of 140 and 141 on a medical image film. However, the level function serves to stretch out the luminance displayed so that the two pixels with a luminance difference of 5 units can be easily differentiated within the leveled medical image.
Window and leveling parameters are most effective when they are tailored to different diagnostic tasks and to radiologist preferences. For example, a physician may want to focus on a specific area within a medical image such as the lung area in a chest x-ray. Typically, selection of window and level values is performed by an apparatus or method that enables a user to manually adjust window and level values within a particular area of interest. This manual method is however tedious and time consuming, especially when a user must select and readjust the window and level settings for each separate area of interest.
Also, in many instances, requirements for viewing medical imagery are time critical. Surgery may be necessary for immediate diagnosis and time is of the essence in such circumstances. It has been determined through medical practitioner usage studies that the main medical imagery workstation tool utilized is the window level transformation. The typical time to review a medical image study when window leveling is required has been determined to be approximately 100 seconds. In contrast, when no window leveling is required, the typical review time drops to 50 seconds.
Accordingly, a system that automatically, adaptively and transparently manipulates defined areas of a medical image and adjusts window and level values to display an image that requires little or no manual manipulation by a medical practitioner is highly desirable.